```
public class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
ListNode head = new ListNode(-1); //head node
ListNode p = head; //working pointer
int carrier = 0;
while(l1!=null || l2!=null){
int num1,num2;
if(l1!=null)
num1 = l1.val;
else
num1=0;
if(l2!=null)
num2 = l2.val;
else
num2 = 0;
int sum = num1+num2+carrier;
if(sum >= 10){
sum = sum % 10;
carrier = 1;
}else{
carrier = 0;
}
ListNode node = new ListNode(sum);
p.next = node;
p = p.next;
if(l1 !=null)
l1 = l1.next;
if(l2 !=null)
l2 = l2.next;
}
if(carrier == 1){//last carrier
ListNode node = new ListNode(1);
p.next = node;
p = p.next;
}
head = head.next;//skip the first node(-1)
return head;
}
}
class ListNode {
int val;
ListNode next;
public ListNode(int x) {
val = x;
next = null;
}
public ListNode(int x,ListNode node){
val = x;
next = node;
}
}
```